Synthetic peptides of the V3 loop region of HIV-1 envelope glycoprotein were used to measure binding antibodies in an ELISA and as blocking agents in a neutralization assay. Samples from three geographic regions, U.S., Zaire, and Brazil were tested in neutralization assays with peptides representative of a portion of the V3 loop sequence of the HIVIIIB, HIVMN, HIVSF2, and HIVZ3 strains to determine if blocking occurs. No peptide blocking was observed using the IIIB and SF2 peptides, minimal blocking was observed using the Z3 peptide and good blocking was observed using the MN peptide. Conclusions about geographic region and peptide blocking relating to strain prevalence in that region are presently being made. Data is currently being summarized in preparation for publication.